Second harmonic generation and photochromic grating in guest-host polymer system containing azo amide chromophores

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1 e-polymers 010, no ISS Second harmonic generation and photochromic grating in guest-host polymer system containing azo amide chromophores Marański K., 1 Kucharski S., 1* rtyl E., 1 Janik R., 1 unzi J.-M., Ahmadi-Kandjani S., 3 Barille R. 1* Wrocław University of Technology, Wrocław, Poland, wroc.pl University of Angers,France, 3 University of Tabriz, Tabriz, Iran (Received: 16 December, 008; published: 16 ovember, 010) Abstract: The azo sulfonamide chromophores containing long aliphatic chain were used as active nonlinear optical (L) materials in films prepared by spin-coating on glass surfaces. The sulfonamides were: -{4-[{4-[(pyrimidin--ylamino)sulfonyl]phenyl}diazenyl]phenyl} decanamide (Amid P); -{4-[{4-[(1,3-thiazol--ylamino)-sulfonyl]phenyl}diazenyl]phenyl} decanamide (Amid T) and -{4-[(4-nitrophenyl)-diazenyl]-phenyl}undec-10-enamide (Amid ). The host matrix was a copolymer of methyl and butyl methacrylate. The formation of diffraction grating by the guest-host system was successful only in the case of Amid. The pitch and amplitude of the grating was 930 nm and ca. 45 nm, respectively. Quantum chemical calculations revealed that the chromophores were moderate L-fores showing static β values of ca m 4 /V. Two procedures were involved to align chromophores to determine first order L susceptibility: a) corona poling, and b) all-optical poling. The L coefficient d 33 determined in corona poled film was 5., 8.6 and 93. pm/v for Amid P, T and, respectively. Introduction Photosensitive substances such as azobenzene derivatives can be characterized as related to the types of substituents on both sides of benzene rings. This makes them interesting by the fact that choice of proper electron donor and electron acceptor group creates a system that can be adjusted to show photochromic and nonlinear optical properties related to expected maximum absorption band. The azobenzene derivatives are known photochromic compounds and on illumination they undergo trans-cis isomerization. This transformation is reversible and back cis-trans reaction can proceed according to thermal relaxation scheme or also on illumination with light absorbed by cis form [1-6]. By proper choice of substituents on both sides of azobenzene molecule one can obtain delocalized П-electron system in acentrosymmetric aromatic molecule leading to materials of high second order nonlinear optical (L) properties [7-11]. When the molecule is subjected to an intense light field, the induced dipole moment is given by: μ = μ + i o 1 α ij E j + β ijk E j E k + j, k 1 6 j, k, l γ ijkl E where μ o is molecular dipole moment, μ i is the ith component of the induced dipole moment and E i are components of the applied electromagnetic field. The coefficients α ij, β ijk and γ ijkl are components of linear polarizability, the first hyperpolarizability and j E k E l (1) 1

2 second hyperpolarizability tensors, respectively. The macroscopic second order L susceptibility, Χ ijk (),is a parameter characterizing material property and is correlated with β ijk as shown by Eq. (): ( ) Χ ijk = F i ( ω 3 ) F j ( ω 1) F k ( ω ) G ( ijk ; xyz ) β ijk () where: is number density of chromophore, F(ω n ) is the local field factor at frequency ω n, G is a function serving to translate molecular coordinates into laboratory ones. To achieve the second order susceptibility it is necessary to make molecular arrangement in bulk materials to get acentrosymmetric electronic distribution on macroscopic scale. There are several methods to do so, like Langmuir-Blodgett technique [1-14], corona poling and all optical poling; the last one can be used with materials of ionic structure [15, 16]. The L chromophore can be used as dissolved in a polymer matrix (guest-host system) or as a side-chain fragments connected to polymer main chain by chemical bonds. The idea of this work was to use chromophoric azo amides with electron donor group at nitrogen atom of the alkanamide type. The presence of carbonyl group at that place (Fig. 1) weakens the donor ability of the nitrogen atom and therefore modifies the absorption ability of the chromophores by shifting maximum absorption band towards UV region. The selected chromophores were yellow solid substances and their synthesis and absorption properties were described in [17, 18]. The aim of the work was to determine photochromic properties of the chromophores in polymer guest-host matrix and their L properties of the second order after molecular alignment of the chromophores was done by corona poling and all-optical poling (AP). Results and Discussion Photochromic amides of the kind being under question and whose structures are shown in Fig. 1, were found to undergo reverse trans-cis-trans isomerization in solution [0]. R H C R1 SYMBL Amid P Amid T Amid R1 -C 9 H 19 -C 9 H 19 -C 7 H 15 CH=CH R H S S H S Fig. 1. Chemical formulae of chromophores.

3 Using a guest-host system in polymer matrix makes it possible to use the dyes in relatively high concentration, and besides, it is possible to formulate the materials in form of thin films on transparent solid surfaces. It is to mention that the dyes in concentration up to 30 wt. % in poly(methyl methacrylate-co-butyl methacrylate) could be used to deposit thin films by spin coating. Formation of transparent photochromic film on transparent support could be very useful in preparation of the devices for practical applications. The absorption maximum of the films was below 400 nm (Tab. 1), still the tail of absorption band lay in the range of 488 nm laser reach and also there was small absorption observed at 53 nm which was frequency doubled beam of 1064 nm laser. Tab. 1. Spectral properties of chromophore containing guest-host films. Type of chromophore λ max [nm] Absorption coefficient [cm -1 ] at 488 nm at 53 nm Amid P Amid T Amid Fig.. Surface relief grating in film containing Amid. The films were equilibrated after spin-coating by keeping them at the temperature above T g of the copolymer matrix for 4 h. The surfaces of the films so prepared were scanned with atomic force microscope (AFM) and average roughness of the surface was within ca.3 nm. The films so prepared were subjected to illumination with two-beam coupling arrangement using a 488 nm laser beam in so called Lloyd mirror [1] to record surface relief grating. Among the amides tested only that containing nitro group (Amid ) cooperated with copolymer matrix. This means that the azo dye in trans form escaping from bright domains of light interference pattern caused the surface movement of copolymer material forming the surface relief grating (Fig. ). Using the AFM helped us to determine its amplitude and pitch which was in the range from ca. 45 to 50 nm, and 950 nm, respectively. The pitch calculated from Lloyd mirror 3

4 geometry was 1040 nm confirming fair agreement between theoretical and experimental data. First order nonlinear susceptibility is a direct result of the first hyperpolarizability that can be observed (calculated or/and determined) on molecular scale level. To develop the first order L effect on macroscopic scale it is necessary to order chromophore molecules in such a way that their dipoles point out in the same direction. This ordering has to be forced as the molecules show tendency to remain in random arrangement. There are several methods to force dipoles ordering in macroscopic materials, and the most important ones, applied mainly to polymeric materials, are corona poling and all-optical poling. The corona poling requires action of electric field of high intensity in materials at temperatures close or higher than glass transition temperature of the polymer in the first step, and cooling the material under applied field, that freezes the alignment of the molecules. In the case of amides in question, the guest-host system containing ca. 30 wt.% of chromophore in polymer matrix was used and the film was formed on glass support by spin coating technique. The thickness of the films was in the range of nm, and SHG signal was recorded 4 h after corona poling alignment. The SHG signal was probed with s- or p-polarized 1064 nm laser beam, and the frequency doubled signal of 53 nm was recorded in p-polarization to determine d 31 and d 33 nonlinear optical coefficients. KTP was used as a standard and the exact procedure of measurements and calculations using films oriented by both corona poling and all-optical poling was described in [0, 1]. If s-polarized fundamental beam was sent to the samples at different incident angles (α) relative to the surface normal, a p-polarized SHG signal was then measured. Under this scheme (s-in/p-out) the effective nonlinear optical coefficient d eff was expressed as: d eff = d 31 sinα (3) In s-in/p-out measurement scheme, d eff depends only on d 31. By fitting the SHG signal data, we thus obtained the value of the L coefficient d 31 which could be introduced to fit the data in p-in/p-out configuration. In the latter configuration, the effective nonlinear optical coefficient d eff depends on both d 31 and d 33 : d eff = d 31(sin α cos α + cosα sin α ) + d 33 sin α sin α sin α (4) ω ω ω ω ω ω ω Fig. 3. Maker fringes SHG signals of the films containing Amid P (upper: film thickness 700 nm) and Amid (lower: film thickness 800 nm). Upper curves: polarization p => p, lower curves: polarization s => p. 4

5 In calculations of guest-host systems the refractive indices at 1064 and 53 nm were assumed as those of host polymer (1.489 and 1.49, respectively). Fig. 3 shows the SHG signal as a function of probing beam incident angle yielding p and s polarization image of known shape called Maker fringes. It is obvious that the p => p polarization system is more efficient. Table contains the results of first order L susceptibility. The best value may be ascribed to a nitro derivative, i.e., Amid, for which the determined d 33 L coefficient was 93. pm/v. For the sulfonamide derivatives, i.e., Amid P and Amid T, the value of d 33 was 5. and 8.6 pm/v, respectively. Taking into account the calculated values of the first hyperpolarizability (Tab. ) one might not expect such discrepancy. However, the reason of it might be found in different molecular structures of the chromophores. The nitro group is known to be much stronger as an electron acceptor than sulfonamide group [9]. Besides, the Amid has a flat chromophore part of the structure contrary to Amid P and Amid T. In both molecules the heterocyclic ring is twisted by ca. 90 degree with respect to phenyl azo part, however taking into account d 31 /d 33 ratio as a measure of molecular alignment, one might come to the conclusion that twisted molecular structure of the sulfonamides is no obstacle for the alignment SHG [a.u.] SHG [a.u.] time [min] time [min] Fig. 4. All-optical poling, SHG signal vs.time; recording period up to 40 min. Upper: Amid T, lower: Amid. In all-optical poling the alignment of chromophores is caused by electromagnetic radiation being a result of interference of probing wave (E ω ) and its second harmonic (E ω ). The intensity of the filed is E(t) = E ω (t) + E ω (t) and it brings about excitation of the molecules whose dipole moments are not perpendicular to polarization direction of the radiation. In this way the centrosymmetry of the material is disturbed and thus the conditions are created to generate the second order L response. Determination of SHG signal by AP was carried out in two stages. The first stage, i.e., the stage of alignment takes place during illumination of film samples with probing beam (1064 nm) and its second harmonic (53 nm) beam. This is the stage of recording, shown in Fig. 4 within first 40 minutes, being a part of plot of growing SHG signal tendency. The second stage, next 40 minutes, was SHG signal recording period when the 53 nm beam was cut off. The values of L coefficients obtained by AP are considerably lower as compared with those obtained by corona poling (Tab.). It is obvious that the mechanism of 5

6 chromophore orientation by AP is different and specific. The amides in question are diazo compounds that undergo trans - cis isomerization of the diazo group. This causes the change of the molecular structure of linear trans isomer into a bent structure of cis form. The latter has considerably lower first hyperpolarizability value. The accompanying 53 beam is absorbed by the chromophore and trans-cis isomerization reduces net first order L susceptibility not only reducing the number of trans molecules but also by molecular movement in materials caused by the change of molecular structure of the chromophore. evertheless, the chromophores of the amide type may be assumed as moderate L-fores due to relatively low absorption at the SHG 53 nm beam. Tab.. The data of nonlinear optical coefficient determined after corona poling and all-optical poling. Type of chromophore d 31 Corona poling d 33 AP d 31 /d 33 d 33 pm/v β o *) (m 4 /V)*10 40 pm/v pm/v Amid P Amid T Amid * calculated by GAMESS with RHF/3-1 basis set []. Conclusions The azo amide chromophores containing long aliphatic chain can be used to form transparent films of the guest-host type in methacrylate copolymers. Among three azo amides in question the one with derivative containing nitro group was found to be photoactive in formation of surface relief grating on illumination with 488 nm laser in Lloyd mirror device. The amides showed first order nonlinear optical susceptibility and the value of d 33 obtained for Amid (with nitro group) equal to 93. pm/v indicates that this compound could be useful chromophore for L applications. Experimental Materials Three chromophores were used to prepare photochromic guest-host films on glass support. The chromophores whose chemical structure is shown in Fig. 1 are: -{4- [{4-[(pyrimidin--ylamino)sulfonyl]phenyl}diazenyl]phenyl}decanamide (Amid P); -{4- [{4-[(1,3-thiazol--ylamino)-sulfonyl]phenyl}diazenyl]phenyl} decanamide (Amid T) and -{4-[(4-nitrophenyl)-diazenyl]-phenyl}undec-10-enamide (Amid ). The synthesis and spectral properties of the chromophores were described elsewhere [17]. The films were prepared using poly(methyl methacrylate-co-butyl methacrylate) of Sigma-Aldrich as a matrix polymer. The composition guest-host contained 30 % chromophore by wt. and the films were prepared by spin coating using 5 % solution of the composition in THF. The film thickness was evaluated using a Dektak 6M Stylus Profiler. 6

7 Surface relief grating (SRG) The 488 line of an Ar + laser was split on a Lloyd mirror into two beams of equal intensity with appropriate polarizations which interfered on the polymer film [1]. The periodicity (Λ) of the resulting grating could be predicted if the angle between writing beams were known. The quarter-wave plate was used to set circular polarization of writing beams. AFM A PicoSPM (Molecular Imaging Corp.) device with contact mode was used to detect the changes of polymer films surfaces after producing SRG. UV-vis UV-vis spectra were recorded using a Perkin Elmer UV/VIS/IR Spectrometer Lambda 19. Second harmonic generation (SHG) The SHG signal was determined by reading frequency doubling signal of the films treated to force alignment of chromophores. The methods used to force chromophore orientation were corona poling and all-optical poling. Exact description of the experimental procedures and calculations of nonlinear optical coefficients was presented previously in [1]. Acknowledgements This work was supported by Socrates program of European Union and in part by grant o of Polish Ministry of Science and Informatics. References [1] Rau, H. in: Photochemistry and Photophysics, Ed. Rabek J.F., CRC, Boca Raton, FL, 1990, Vol. II, Ch. 4, p [] atansohn, A.; Rochon, P. Chem.Rev., 00, 10, [3] Delaire, J.A.; akatani, K. ibid., 000, 100, [4] Kawata, S.; Kawata, Y. ibid., 000, 100, [5] liveira,.. Jr.; in: Sekkat, Z.; Knoll, W. (Eds.) Photoreactive organic thin films, Academic Press, 00, Chap. 14, p. 49 [6] Sekkat, Z.; Knoll, W.; in Advances in Photochemistry, eckers, D.C.; Volman, D.H.; Bűnau, G.; (Eds.), Vol,, p. 117, Academic Press, [7] Bosshard, C.; Sutter, K.; Prêtre, P.; Hulliger, J.; Flőrsheimer, M.; Kaatz, P. rganic onlinear ptical Materials, Gordon and Breach, Basel, [8] alva, H.S.; Miyata, S. (Eds.), onlinear ptics of rganic Molecules and Polymers, CRC Press, Boca Raton, FL, [9] Kanis, D.R.; Ratner, M.A.; Marks, T.J. Chem. Rev.,1994, 94,19. [10] Yesodha, S.K.; Pillai,C.K.S.;Tsutsumi,. Prog. Polym. Sci. 004, 9, 45. [11] Messier, J.; Kajzar.; Prasad, P..; Ulrich, D. rganic Molecules for onlinear ptics; Kluwer: Dordrecht, [1] Ashwell, G. J.; Jefferies, G.; Dawney, E. J. C.; Kuczynski, A. P.; Lynch, D. E.; Gongda, Y.; Bucknall, D. G. J. Mater. Chem. 1995, 5(7),

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